A fuel cell is an energy conversion device directly converting chemical energy of fuel into electric energy. In other words, a fuel cell employs a power generation method utilizing a fuel gas and an oxidizer, and using electrons generated during an oxidation-reduction reaction thereof to produce power. A membrane-electrode assembly (MEA) of a fuel cell is a part where an electrochemical reaction of hydrogen and oxygen occurs, and is formed with a cathode, an anode and an electrolyte membrane, that is, an ion conductive electrolyte membrane.
A redox flow battery (oxidation-reduction flow battery) is a system charged and discharged by active materials included in a liquid electrolyte being oxidized and reduced, and is an electrochemical storage device directly storing chemical energy of the active materials as electric energy. A unit cell of the redox flow battery includes an electrode, an electrolyte and an ion-exchange membrane (electrolyte membrane).
Due to their high energy efficiency and environmental friendly properties of low contaminant emissions, fuel cells and redox flow batteries have been researched and developed as a next generation energy source.
A core constituent in a fuel cell and a redox flow battery is a polymer electrolyte membrane capable of cation exchange, and properties of 1) excellent proton conductivity, 2) preventing an electrolyte crossover, 3) high chemical resistance, 4) strengthening mechanical properties and/or 4) low swelling ratio are favorably required. The polymer electrolyte membrane is divided into fluorine-based, partial fluorine-based, hydrocarbon-based and the like, and a partial fluorine-based polymer electrolyte membrane has advantages of exhibiting excellent physical and chemical stability and high thermal stability by having a fluorine-based main chain. In addition, in the partial fluorine-based polymer electrolyte membrane, a cation transfer functional group is attached at the end of the fluorine-based chain as in a fluorine-based polymer electrolyte membrane, and therefore, advantages of both a hydrocarbon-based polymer electrolyte membrane and a fluorine-based polymer electrolyte membrane are capable of being obtained.
However, a partial fluorine-based polymer electrolyte membrane has a problem in that cation conductivity is relatively low since micro-phase separation and aggregation of a cation transfer functional group are not effectively controlled. Accordingly, researches have been progressed in the direction of securing high cation conductivity through controlling distribution and micro-phase separation of a sulfonic acid group.